1. Field of the Invention
The present invention is related to a bearing unit for wheel with speed sensor which supports an automobile wheel so that the wheel rotates freely with respect to the suspension device, and detects the rotation speed (rpm) of the wheel.
2. Description of the Prior Art
A rolling-bearing unit with a speed sensor, such as that disclosed in U.S. Pat. No. 4,968,156, has previously been known in which the rolling-bearing unit supports an automobile wheel so that the wheel rotates freely with respect to the suspension device, and which detects the rotation speed of the wheel in order to control the the anti-lock brake system (ABS) or traction control system.
The rolling-bearing unit with speed sensor disclosed in the above patent is constructed as shown in FIGS. 1 and 2. A pair of inner rings or races 2 which are coupled together by a coupling ring, are fitted around the wheel axle 3. Both of the inner rings or races 2 are held and secured between a holder 4 and nut 5. An inner ring raceway 6 is formed around the outer surface of each of the inner rings or races 2.
Also, a double row of outer ring raceways 8 are formed in the inner circumferential surface of a hub 7 which functions as the outer ring or race. Several balls 9 are located between each of the outer ring raceway 8 and the inner ring raceway 6, so that the hub 7 is freely rotatably supported around the wheel axle 3. The wheel 10 is attached to a flange 11 formed on the outer circumferential surface of the hub 7.
Furthermore, there is a seal ring 12 comprising a seal material 16 and a core metal ring 13 which is fitted into and fastened to the opening on the axially inside end of the hub 7 (the axially inside end is the end on the right in FIG. 1 which faces toward the widthwise center of the vehicle when installed). Also, a tone wheel 14 is fastened to this core metal ring 13. This tone wheel 14 is made up of permanent magnets and is arranged so that the S-poles and N-poles alternate around the circumference.
There is a support ring 15 that is fitted around and fastened to the axially inside end of the axially inside one of the pair of inner rings or races 2. The edge of the seal material 16 is formed so that it comes in contact with the inner circumferential surface and axially outside surface of this support ring 15, so that it prevents dust and rain water from getting into the area where the balls 9 are located. Also, there is a sensor 17 supported by part of the support ring 15, and the detection section of this sensor 17 faces the axially inside surface of the tone wheel 14 through an opening in the support ring 15.
In the rolling-bearing unit with speed sensor described above, the wheel 10, which is fastened to the hub 7, can be supported so that it turns freely with respect to the wheel axle 3 around which the inner rings or races 2 are supported. Moreover, the sensor 17 faces the axially inside surface of the tone wheel 14 fastened to the hub 7, and when the hub 7 turns together with the wheel 10, the output of the sensor 17 changes. The frequency at which the output from the sensor 17 changes is proportional to the rpm of the wheel, and if the signal output from the sensor 17 is input to a controller (not shown in the figure), it is possible to find the rpm of the wheel and adequately control the anti-lock brake system (ABS) and traction control system (TCS).
In the rolling-bearing unit with speed sensor of this invention, by completely preventing the sensor 17 to turn around the wheel axle 3, it is possible to prevent failures that occur when the lead wire 18 used for fetching the output signal from the sensor 17 is cut.
In the construction shown in FIG. 1, rotation of the inner rings or races 2 around the wheel axle 3 is prevented by the friction force that occurs between the inner circumferential surface of the inner rings or races 2 and the outer circumferantial surface of the wheel axle 3, and by the friction force that occurs between the axial ends of the inner rings or races 2 and axially outside surface of the holder 4 and axially inside surface of the nut 5. If the rolling-bearing unit is used for long periods of time, however, the friction forces mentioned above gradually decrease and the inner rings or races 2 are caused to circumferentially move or turn around the wheel axle 3 by the rolling friction occurring between the inner ring raceways 6 and the balls 9, which is so called as "creep".
In normal rolling-bearing units which is not equipped with such a speed sensor, there is no real problem even if the creep occurs. However, if the creep occurs in a rolling-bearing unit with speed sensor, the support ring 15 and sensor 17 also turn around the wheel axle 3 together with the inner ring or race 2, and the lead wire 18 attached to the sensor 17 is pulled very hard. There is also a strong possibility that the lead wire 18 will get wound around the hub 7 and be cut.
If the lead wire 18 is cut, it is no longer possible to detect the rpm of the wheel, and thus it is not possible to control the anti-lock brake system and traction control system. Of course, it is possible to prevent the inner rings or races 2 from turning due to the rolling friction if the nut 5 is extremely tightened. However this is not always possible because the tightening force of the nut 5 could not be increased due to the preload pressure on the balls 9, and the nut 5 itself may become loose if used for long periods of time. Therefore, it is difficult to expect that this will effectively stop the inner rings or races 2 from turning.
It is also possible to consider using a spline joint or key joint between the inner circumferential surface of the inner ring or race 2, which supports the sensor 17, and the outer circumferential surface of the wheel axle 3, to make relative rotation impossible. However it is not too desirable because the work involved in forming a spline groove or key groove is very troublesome causing the manufacturing costs to be increased.
FIG. 3 shows construction as disclosed in U.S. Pat. No. 4,946,296, where the sensor 17 is prevented from turning around the wheel axle (omitted from FIG. 3, but see the wheel axle 3 of FIG. 1).
A knuckle spindle 119 is formed at the end portion of the wheel axle onto which the inner ring or race 2 is fitted. The knuckle spindle 119 is formd with a recessed section 119a at a portion on its outer circumferential surface, while the support ring 15 with the sensor thereon has a main body fitted onto the inner ring or race 2 and a projected section 15a on its inner circumferential surface. The projected section 15a is engaged with the recessed section 119a so as to prevent the support ring 15 from rotating with respect to the wheel axle 3.
In the construction as mentioned above, the sensor 17 is not rotated around the wheel axle 3, and threfore the lead wire 18 attached to the sensor is not cut. However, the inner ring or race 2 is not tailored to be relatively stationary with respect to the wheel axle 3, resulting in that the sensor 17 is not completely supported with respect to the wheel axle 3. Once the inner ring or race 2 is placed under a creep state with respect to the wheel axle 3, the recessed section 119a is inclined to be easily disengaged from the projected section 15a, and in addition, the sensor 17 is subject to vibrations, resulting in that the wheel rotation speed could not be precisely detected.
The rolling-bearing unit with speed sensor of this invention takes these circumstances into consideration.